1. Field of Invention
The present invention relates generally to optical communications systems and particularly to label distribution protocols in an optical transmission system using wavelength division multiplexing (WDM).
2. Related Art
Current optical networks allow high bandwidth data communications. The transport capacity required to accommodate the growth of communications traffic is provided by optical links using wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) having increased capacity and longer reach. High speed data can be modulated on light waves and transmitted through the optical network. Wavelength division multiplexing (WDM) is a technique for modulating electrical data signals carrying the information of interest on distinct light wave carriers (or channels) having different wavelengths.
D/WDM networks use signaling and routing protocols for rapidly setting up end-to-end connections. Optical cross-connects (OXC) are used in D/WDM networks as a platform for functional integration and network management. OXCs using wavelength routing and signaling protocols are considered fast wavelength switches having more stringent speed, timing and control requirements compared to conventional OXCs, allowing fast end-to-end connectivity. However, current OXC's are in fact hybrid network elements, in that conversion to the electrical domain is necessary.
In theory, a wavelength can be used end-to-end (from source to, destination) across the optical network. However, in practice, to achieve long reach and to avoid wavelength blocking, a wavelength may change through wavelength translation (or conversion). Not all OXC's are capable of wavelength conversion. Optical switches technology (photonic switches) emerge in the transport network. A photonic switch must be able to effect add/drop and switching in the optical domain, routing the signal from the input to output ports entirely in the optical domain.
Another requirement for DWDM networks is to respond quickly to unpredictable traffic intensities and patterns. The Optical Internet is developing towards the optical layer eventually being directly responsive to the IP service layer according to changing traffic situations. To achieve a unified packet and optical switched network architecture, standard routing and signaling protocols may be adapted to the specific requirements of the wavelength routed networks. Known standard signaling and routing protocols are OSPF (open shortest path first), IS-IS (intermediate system-intermediate system), PNNI (private network-network interface), and MPLS/LDP (multi-protocol label switching/label distribution protocol). The signaling system seven (SS7) used in voice networks may also be considered.
The multi-protocol label switching (MPLS) is a network technology intended to deliver traffic engineering capability and QoS (quality of service) performance for carrier networks to support differentiated services. MPLS is currently used with the asynchronous transmission mode (ATM). Examples of labels used with various protocols are DLCI (data link connection identifier) label that travels with the frame relay protocol, “timeslot” for the time division multiplexing (TDM) protocol, or logical channel number (LCN) for X25 protocol.
MPLS can deliver control and performance to IP packets through the use of label switched paths (LSPs), by combining label-swapping with network layer (layer-3) routing. The labels effectively define the LSP in the MPLS domain to carry the packets. The basic idea is to assign short fixed labels to packets at the ingress to an MPLS domain. A major component of the MPLS is the IP routing protocol (OSPF, BGF) that runs on all MPLS capable nodes, at the edge and the core of label switch routers (LSR's).
Other protocols of the MPLS are IP forwarding at the edge LSRs, and label forwarding at interior LSR's. In the MPLS domain, the labels are used to make forwarding decisions, without use of the packet header. Connectivity is captured in the routing database by the routing protocols, while link local labels are assigned for each route, or aggregates of routes for each hop.
The label switched path (LSP) can be manipulated and managed by the network administrator to direct the traffic. The route for a given LSP can be established in two ways: control driven (also called hop-by-hop LSP), or explicitly routed (ER-LSP). Another way for routing an end-to-end routing in a communications network is broadcasting. Broadcasting data packets implies sending a message from a source node to all nodes in the network, without providing directions. This type of routing however is not considered here.
When setting up a hop-by-hop LSP, each label switch router (LSR) determines the next interface to route the LSP based on its layer-3 routing topology database, and sends the label request to the layer-3 next hop. The label information is distributed by a label distribution protocol (LDP).
When setting up ER-LSP, the route for the LSP is specified in the set-up message itself, and this route information is carried along the nodes the set-up message traverses. All the nodes along the ER-LSP will follow the route specification and send the label request to the next indicated interface. In this case, the label information is distributed by a constraint-based routing CR-LDP, which is an extension of the LDP by including an explicit path. The CR-LDP is an efficient solution for core network traffic engineering as regarding the quality of service (QoS) guarantees, path optimization, and flexibility.
While the hop-by-hop LSP follows the path that normal layer-3 routed packets will take, the ER-LSP can be specified and controlled by network operators or network management applications to direct the network traffic, independent of the layer-3 topology.
CR-LDP signaling builds on the existing LDP protocol and provides ER-LSP set-up with optional resource reservation in a simple hard state control and messaging manner. The LDP mechanism by which LSP are created is the same for both hop-by-hop and explicit routes (UDP for peer discovery and TCP for session, advertisement and messaging). The basic LDP protocol is extended to incorporate the explicit route information, the traffic parameters for resource reservation, and the necessary options for ER-LSP reliability and resiliency. An explicit route is represented in a label request message as a list of nodes or group of nodes along the constraint-based route. If the requested path can satisfy the resource required, labels are allocated downstream and distributed by means of label mapping messages.
Using the above techniques, one can imagine sending a messenger ahead of the traffic to reserve capacity for the transmitted data, and for distributing instructions at each node indicating where the packet has to go.